The present invention relates to a process for removing impurities from flue gas streams utilizing ozone. More particularly, the present invention provides for means to remove nitrogen oxide, sulfur oxide and mercury from flue gas streams.
Recent federal and local environmental laws require very significant reduction of discharge of harmful gaseous substances into the atmosphere. Chief among such harmful air pollutants are nitrogen oxides (NOx). In response to strict enforcement efforts of these laws, industrial air polluters have made considerable efforts to reduce the amount of these harmful substances into the air in gaseous effluents from industrial or municipal sources. Successful efforts to reduce the concentration of NOx in gaseous effluents often involve reacting the NOx in waste gases with nitrogen-based reducing agents. One commercially used method of reducing NOx from gas streams involves contacting the NOx with ammonia or an ammonia precursor, such as urea, in the absence of a catalyst, a technique known as selective non-catalytic reduction (SNCR). The ammonia reduces the NOx to nitrogen while itself being oxidized to nitrogen and water. SNCR processes require very high temperatures, for instance temperatures in the range of about 800 to 1200xc2x0 C., and even at these temperatures only low conversions of NOx are achieved. For example, it is not uncommon to attain NOx reductions only in the range of 40 to 50% by SNCR-based processes.
Another technique for removing NOx from waste gas streams involves contacting the waste gas with ammonia or an ammonia precursor in the presence of a substance which catalyzes the reduction of NOx to nitrogen, as in SNCR processes. These catalytic reduction processes are referred to as selective catalytic reduction (SCR). SCR processes have a few advantages over SNCR processes. They can be carried out at temperatures significantly lower than the temperatures at which SNCR processes are carried out. For example, they are quite effective at temperatures in the range of about 250 to 600xc2x0 C. Although SCR processes are more efficient than SNCR processes in the reduction of NOx to nitrogen, they have the disadvantages of being more costly than SNCR processes, the catalyst can be poisoned or deactivated and often they do not remove all of the NOx from the gas stream being treated.
Another disadvantage of both SCR and SNCR processes is that ammonia, which itself is regarded as an environmentally unacceptable pollutant, is often released into the atmosphere in the gaseous effluent from the reactor because the reactions are often conducted in the presence of excess ammonia and/or because of sudden changes in the process that produces less than expected NOx. Ammonia may also be released because of depletion or masking of the catalyst by contamination over time.
Another known method of removing NOx from gas streams involves contacting the NOx with ozone or with oxygen and hydroxyl radicals generated with barrier discharge, thereby oxidizing them to higher nitrogen oxides, such as N2O5 and removing the higher oxides from the gas stream by means of aqueous scrubbers.
Specific details of ozone-based NOx oxidation processes are disclosed in U.S. Pat. Nos. 5,206,002 and 5,316,737, the disclosures of which are incorporated herein by reference. Ozone-based NOx oxidation processes can be expensive because of the high cost of producing ozone and require efficient use of ozone to reduce costs.
Barrier discharge technology is disclosed in U.S. Pat. Nos. 5,871,703 and 6,117,403. However the efficiency of this technology is relatively low, about 70 to 80% for NOx, 40 to 50% for SOx and 60 to 85% for mercury.
Because of stringent environmental regulations, efforts are continuously made to improve NOx removal processes to minimize or eliminate emission of NOx into the atmosphere. Additionally, the Environmental Protection Agency has presented information showing mercury levels in the environment are at levels that are likely to lead to adverse health effects. Coal-fired utility boilers are one of the largest sources of harmful anthropogenic mercury emissions but also include the NOx, and SOx emissions sources. The present inventors have discovered a process whereby by improving the barrier discharge technology, mercury as well as NOx and SOx can be removed in excess of 95% from the emission from coal-fired utility boiler flue gas.
The present invention provides for a process for removing nitrogen oxides, sulfur oxides and mercury from a gas stream comprising the steps of (1) feeding the gas stream into a dry electrostatic precipitator to remove dust and particulates from the gas stream; (2) contacting the gas stream with ozone, oxygen and hydroxyl radicals generated in a barrier discharge; (3) contacting the gas stream with ozone in a reaction zone to convert the nitrogen oxides to nitric acid, nitric acid precursors and mixtures thereof, and to convert the mercury to mercuric oxide; and (4) feeding this gas stream into an aqueous scrubber thereby removing the nitric acid, nitric acid precursors, sulfur oxides and the mercuric oxide resulting in a cleaner gas stream for emission to the atmosphere.